Olefin polymerization catalysts comprising transition metal compounds and organometallic compounds have been heretofore known as catalysts for preparing olefin (co)polymers such as ethylene polymer, propylene polymer and ethylene-.alpha.-olefin copolymer. Above all, olefin polymerization catalysts comprising transition metal compounds such as zirconocene and organoaluminum oxy-compounds (aluminoxane) are known as catalysts capable of preparing olefin (co)polymers with high polymerization activity. Processes for preparing olefin (co)polymers using such catalysts have been proposed in, for example, Japanese Patent Laid-Open Publications No. 19309/1983, No. 35005/1985, No. 35006/1985, No. 35007/1985 and No. 35008/1985.
Further, processes for polymerizing olefin in a suspension polymerization system or in a gas phase polymerization system using solid catalysts in which at least one component of a transition metal compound component and an organoaluminum oxy-compound component is supported on a porous inorganic oxide carrier such as silica, alumina or silica alumina have been proposed in, for example, Japanese Patent Laid-Open Publications No. 35006/1985, No. 35007/1985 and No. 35008/1985. Furthermore, processes for polymerizing olefin in the presence of solid catalysts in which a transition metal compound such as metallocene and aluminoxane are supported on a carrier such as inorganic oxide are described in Japanese Patent Laid-Open Publication No. 108610/1986 and No. 296008/1986.
Moreover, a process for prepolymerizing olefin in the presence of a zirconocene compound, aluminoxane, an organoaluminum compound and a carrier such as silica is described in Japanese Patent Laid-Open Publication No. 280703/1988.
By the way, in the case where olefin (co)polymers are prepared using the solid catalysts or prepolymerized catalysts comprising transition metal compounds, organoaluminum oxy-compounds and carriers mentioned above, there sometimes arises a problem that polymers having particle diameters of not more than 100 .mu.m (fine-powdery polymers) are produced in large amounts or polymers of excellent particle properties are hardly obtained.
In the case where propylene (co)polymers are prepared using such the catalysts or prepolymerized catalysts comprising transition metal compounds, organoaluminum oxy-compounds and carriers mentioned above, particle properties of the resulting polymers are not always good, and the polymers sometimes stick to the wall of the polymerization reactor. Additionally, the resulting polymers sometimes show poor mechanical properties because a component having low molecular weight is contained in a large amount.
The present inventors have earnestly studied under such circumstances as mentioned above, and they have found that the properties of a solid catalyst or a prepolymerized catalyst have influence on the properties of the resulting polymer and that the molar ratio of the alkyl group to the aluminum atom in the organoaluminum oxy-compound have influence on the activity of olefin polymerization and the properties of the resulting polymer.
In this connection, the present inventors have further studied and found that a solid catalyst or a prepolymerized catalyst, which has a specific bulk density and a specific fluidity index, and a solid catalyst or a prepolymerized catalyst, which comprises an organoaluminum oxy-compound having a specific molar ratio of the alkyl group to the aluminum atom, a fine particle carrier and a transition metal compound and has a specific bulk density and a specific fluidity index, hardly produce a fine-powdery polymer in the olefin polymerization procedure and can prepare an olefin polymer of excellent particle properties.
The present inventors have also found that a solid catalyst, which comprises an organoaluminum oxy-compound having a specific molar ratio of the alkyl group to the aluminum atom, a fine particle carrier and a specific transition metal compound, has high activity of propylene polymerization, hardly produces the component of low molecular weight and can prepare a polymer of excellent particle properties. Based on these findings, the present invention has been accomplished.